Generally, it is desirable for an internal combustion engine to maintain a stable idle speed. By maintaining a stable idle speed, stalling of the engine due to load, or torque, variations can be minimized.
The prior art teaches many methods of idle speed control. For example, U.S. Pat. No. 4,719,573, issued to Kissel et al., discloses a method for idle speed control for an engine equipped with a fuel/air mixture means and an air bypass means. The spark advance is temporarily changed in response to idle speed changes due to changing engine load, either by retarding or advancing the spark advance value. Thus, the idle speed is moved to the targeted value and held there until the air bypass system has a chance to respond. As the air bypass valve is properly positioned, the spark advance is changed back to a steady state value, allowing the idle speed actuator valve or motor to operate in a stable manner without the undesirable effects of slow response.
U.S. Pat. No. 4,696,272, issued to Kato et al., discloses an ignition timing control method for internal combustion engines. The method is adapted to control the timing of the ignition of a mixture being supplied to the engine and includes the steps of retarding a determined ignition timing value by a predetermined amount when the engine is in a predetermined idling region and the engine temperature exceeds a predetermined value, and setting a desired idling speed in response to operating conditions of the engine. The method also includes the steps of calculating a correction value by multiplying the difference between a desired idle speed and an actual engine speed by a predetermined coefficient, and correcting the ignition timing value retarded by the predetermined amount by the correction value thus calculated.
U.S. Pat. No. 4,509,477, issued to Takao et al., discloses a method and apparatus for idle operation control of internal combustion engines. The control of the rotational speed of the engine to a predetermined value during idle operation is performed by detecting an engine rotational speed variation characteristic, detecting an engine torque variation characteristic and detecting a phase difference between the two characteristics on the basis of the engine rotational speed. An engine torque variation condition is determined at the present moment on the basis of the torque variation characteristic data and the phase difference data and an engine operation parameter, such as ignition timing, is then corrected in a direction to minimize the engine torque variation.
Other patents related to controlling the idle speed of an internal combustion engine include U.S. Pat. No. Re. 33,027, issued to Danno et al., U.S. Pat. No. 5,033,432, issued to Ohuchi et al., U.S. Pat. No. 5,024,197, issued to Nakamura, U.S. Pat. No. 5,016,180, issued to Fujisawa, U.S. Pat. No. 4,933,863, issued to Okano et al., U.S. Pat. No. 4,890,592, issued to Furuyama et al., U.S. Pat. No. 4,883,034, issued to Yashiki et al., U.S. Pat. No. 4,879,656, issued to Quigley et al., U.S. Pat. No. 4,676,211, issued to Kiuchi et al. and U.S. Pat. No. 4,552,109, issued to Boccadoro et al.
Previous strategies, however, utilized only one or two fixed values of spark retard, restricting the degrees of freedom of these strategies and only crudely approximating the complex behavior of spark advance as a function of speed and load.